Are you gonna have enough outside wall space to have 3 intake and 3 exhaust pipes? To exhaust with PVC requires expensive condensing boilers. Burnham and Weil Mclain make cast iron wall mounted direct vent boilers (stainless vents) in the small outputs you're looking for. Is using the old chimney no longer an option? There are several less expensive small natural vent cast iron boilers out there you could get for the tenant aprtments.

Takagi does make the Jr. model that's listed as approved for heating but I'm not sure it would modulate down that low and might start short cycling if just used for a heating loop.